Titanium alloys are frequently used in aerospace and aeronautical applications because of their superior strength, low density, and corrosion resistance. Titanium and many titanium alloys exhibit a two-phase behavior. Pure titanium exists in an alpha phase having a hexagonal close-packed crystal structure up to its beta transus temperature (about 1625° F.). Above the beta transus temperature, the microstructure changes to the beta phase, which has a body-centered-cubic crystal structure. Pure titanium is unduly weak and too ductile for use in most aerospace and aeronautical applications, though. To achieve the necessary strength and fatigue resistance, titanium is typically alloyed with other elements.
Certain alloying elements may affect the behavior of the crystal structure, allowing the beta phase to be at least metastable at room temperature. Alpha-beta alloys are typically made by adding one or more beta stabilizers, e.g., vanadium, that inhibit the transformation from beta to alpha and allow the alloy to exist in a two-phase alpha-beta form at room temperature.
The two most prevalent titanium alloys in use in aerospace and aeronautical applications are likely Ti 64 and Ti 6242. Both of these alloys are titanium-based alloys, i.e., at least about 50% of the alloy comprises titanium. Ti 64 is an alpha-beta alloy that consists principally of about 6 weight percent (wt. %) aluminum, 4 wt. % vanadium, and the balance titanium and incidental impurities. Ti 6242 is also an alpha-beta alloy and it consists principally of about 6 wt. % aluminum, 2 wt. % tin, 4 wt. % zirconium, 2 wt. % molybdenum, and the balance titanium and incidental impurities.
Beta and alpha-beta titanium alloys are known to be sensitive to the cooling rate when cooled from a temperature above the beta transus temperature. FIG. 1 is photomicrograph (taken at 200× magnification) of a beta-annealed Ti 64 plate. FIG. 2 is a photomicrograph (also taken at 200× magnification) of a Ti 6242 casting. Both of these microstructures exhibit a relatively coarse “basketweave” of alpha and beta crystals. The basketweave is coarser in the Ti 6242 alloy (FIG. 2). Alpha phase is also precipitated at the grain boundaries in both alloys during cooling. This alpha precipitation significantly decreases ductility and reduces fatigue strength of the alloy.
To achieve a commercially acceptable titanium alloy, it is well known in the art that the alloy must be cooled very quickly to limit the precipitation of alpha phase at the grain boundaries. For this reason, conventional wisdom dictates that beta and alpha-beta alloys such as Ti 64 and Ti 6242 must be quenched rapidly if heated to or above the beta transus temperature. Typically, the rapid cooling is at least as fast as air cooling. Alpha-beta titanium alloys are also frequently cooled even faster, e.g., with a gas, water, or oil quench. It has been suggested that cooling rates in the range of 700-1200° F. per minute are optimal to maintain creep and low-cycle fatigue of alpha-beta Ti 6242S (which comprises Ti 6242 with the addition of a minor fraction, e.g., 0.09 wt. %, of silicon), for example. (See, e.g., U.S. Pat. No. 5,698,050, the entirety of which is incorporated herein by reference.)
Even if titanium alloys are heated to a temperature below the beta transus temperature, common knowledge dictates that the alloy should be cooled rapidly to maintain acceptable mechanical properties. For example, the United States Department of Defense has published specifications for the heat treatment of titanium alloys under Military Specification MIL-H-81200B, the entirety of which is incorporated herein by reference. In this military specification, all beta and alpha-beta titanium alloys are air-cooled, cooled with an inert gas, or quenched with water or oil; furnace cooling is specifically prohibited. The specifications further set forth maximum delay times of 10 seconds or less to initiate quenching to avoid undue precipitation of grain boundary alpha phase. Aerospace Material Specification AMS 4919B provides similar admonitions regarding cooling rates for beta and alpha-beta titanium alloys.
The need to rapidly quench beta and alpha-beta titanium alloys can limit their use in some structural applications. For example, the properties of alpha-beta titanium alloys can drop off significantly as the thickness of a cast or forged part increases. This is due, at least in part, to the differential cooling rate between the outer portions and the inner portions of the formed structure. For Ti 64 alloys, for example, the tensile strength and fracture resistance for cast or forged parts drops significantly in areas having a thickness of five inches or more. To compensate for the drop-off in mechanical properties, the thick parts of a cast or forged Ti 64 member must be made even thicker, both exacerbating the cooling rate difficulties and increasing the weight and cost of the final finished part.